1. Field of the Invention
The present invention relates generally to a pulse conditioning circuit and, more particularly, to an improved pulse conditioning circuit in battery-powered flow meters for conditioning input pulses and minimizing power consumption.
2. Description of the Prior Art
Battery-powered, natural gas flow meters, such as those sold by American Meter Company, calculate volumetric flow by compensating raw volume data based upon temperature and pressure measurements. The raw volume data is typically determined by monitoring a pulse output from a volumetric meter placed in the natural gas pipeline. The meter may have an impeller or other mechanical means (such as a diaphragm and chamber system), which turns or cycles as the natural gas flows through the meter. The impeller or mechanical means is connected to a shaft which turns as the natural gas flows through the meter. Most meters include a mechanical index connected to the shaft to provide a mechanical history of the gas flow.
Generally, the shaft also includes one or more magnets oriented to pass within close proximity of one or more reed switches fixed in the index. The reed switch or switches are electronically arranged to provide pulses to conditioning circuitry as the magnet(s) on the index shaft rotates by the reed switch(es). These pulses provide the electronic flow meter raw flow data for electronic compensation. Problems with prior electrical conditioning circuits include miscount due to switch bounce when the magnet-switch proximity is such to cause a premature switch closure, meter stoppage and shaft reversal, and interference or noise. Noise is of utmost concern in high-impedance, low power applications inherent in battery powered meters. Additionally, prior art circuit designs fail to minimize current consumption to the extent of the present invention, thereby reducing battery life.
In one prior art solution, a single reed switch design has been used for conditioning a switch closure using a passive low pass filter into a gate with hysteresis. However, the single reed switch design does not prevent or eliminate miscount due to meter stoppage and reversal. Further, the single reed switch design allows current to continue to flow through a resistor in the conditioning circuit as long as the reed switch is closed, thereby allowing unnecessary current consumption, which reduces battery life (see FIG. 1).
Another prior art solution employs a dual reed switch design with cross-coupled feedback without filter capacitors. This basic circuit topology provides a "set/reset" flip-flop such that the flip-flop can be set only after it has been reset. The set/reset actions correspond to the consecutive closings of the two reed switches. While the dual reed switch design prevents some miscount due to meter stoppage and reversal, it does not eliminate miscount due to RF interference (see FIG. 2). If filter capacitors are added to improve interference performance, the capacitors act to slow down the latching action of the flip-flop. Thus, if a switch does not stay closed long enough to allow the flip-flop to latch, a miscount may occur (see FIG. 2).
Thus, there remains a need for an electrical pulse conditioning circuit for battery powered flow meters which (a) reliably counts the number of rotational pulse input signals so that the data may be saved for later use, (b) prevents miscount due to meter stoppage and reverse rotation, (c) prevents miscount due to interference or noise from high frequencies, and (d) consumes a minimum current to maximize battery life.